Under certain conditions, some materials can be plastically deformed without rupture well beyond their normal limits, a property called superplasticity. This property is exhibited by certain metals and alloys, within limited ranges of temperature and strain rate. For example, titanium and its alloys are superplastic in the temperature range 1450.degree.-1850.degree. F. (787.degree.-1010.degree. C.).
Superplastic forming (SPF) is a fabrication technique that relies on superplasticity. A typical SPF process involves placing one or more sheets of metal or plastic in a die, heating the sheets to an elevated temperature within the superplastic range, and superplastically forming the sheet(s) at the SPF temperature. Generally, a differential forming pressure from a gas manifold is used to stretch the sheet(s) into the desired shape against the die surface(s). The differential pressure is selected to strain the material at a strain rate that is within its superplastic range. The following patents are illustrative of SPF processes and equipment:
______________________________________ PATENT TITLE ISSUE DATE ______________________________________ 3,920,175 Method of SPF of Metals with November 18, 1975 Concurrent Diffusion Bonding 3,927,817 Method for Making Metallic December 23, 1975 Sandwich Structures 3,605,477 Precision Forming of Titanium September 29, 1971 Alloys and the Like by Use of Induction Heating 4,141,484 Method of Making a Metallic February 27, 1979 Structure by Combined Flow Forming and Bonding 4,649,249 Induction Heating Platen March 10, 1987 for Hot Metal Working 4,117,970 Method for Fabrication of October 3, 1978 Honeycomb Structures 5,024,369 Method to Produce Super- June 18, 1991 plastically Formed Titanium Alloy Components ______________________________________
These patents are incorporated by reference.
One advantage of SPF is that complex shapes can be formed from sheet-metal so the time and waste of milling are eliminated with great cost saving. In addition, the SPF process is generally applicable to single and multisheet fabrication. For multisheet fabrication, SPF is combined with joining processes, such as diffusion bonding or laser welding, to produce complex sandwich structures. One advantage of the SPF process is lighter, lower cost parts with fewer fasteners. A single part can replace the complex assembly currently required using conventional manufacturing operations. Common applications of SPF include the manufacturing of parts for aircraft, missiles, and spacecraft.
In a typical prior art SPF process for titanium or its alloys, the sheetmetal is placed between dies at least one of which has a contoured surface corresponding to the shape of the product. The dies are placed on platens which are heated, generally using embedded resistive heaters. The platens heat the dies to about 1650.degree. F. (900.degree. C.). Because the titanium will readily oxidize at the elevated temperature, an inert gas, such as argon, surrounds the die and workpiece. The dies heat the sheetmetal to the temperature range where the sheetmetal is superplastic. Then, under applied differential pressure, the sheetmetal deforms against the contoured surface.
The platens and dies have a large thermal mass. To save time and energy, they must be held near the forming temperature throughout a production run (i.e., the production of a number of parts using the same dies). The raw sheetmetal must be inserted onto the dies, and formed parts removed, at the elevated forming temperature. The hot parts must be handled carefully to minimize bending. Within the SPF range, the SPF metals have the consistency of taffy, so bending can easily occur unless precautions are taken.
As described to some degree in U.S. Pat. No. 4,622,445 and in copending U.S. patent application Ser. No. 07/777,739, we have discovered an improvement for an SPF process coupling the use of ceramic dies with inductive heating. With this SPF press or workcell, we can heat preferentially the sheetmetal workpiece with induction heating without heating the platens or dies significantly and can use the ceramic dies as an insulator to hold the induced heat in the part. We can stop the heating at any time and can cool the part relatively quickly even before removing it from the die. We do not waste the energy otherwise required to heat the large thermal mass of the platens and dies. We do not force the workers to work around the hot dies and platens. With our inductive heating workcell, we also save time and energy when changing dies to set up to manufacture different parts because the dies and platen are significantly cooler than those in a conventional SPF press. This change operation can be shortened by several hours.
U.S. Pat. Nos. 3,920,175 and 3,927,817 describe typical combined cycles for SPF forming and diffusion bonding. Diffusion bonding is a notoriously difficult and temperamental process that has forced many SPF fabricators away from multisheet manufacturing or to "clean room" production facilities and other processing tricks to eliminate the possibility of oxidation in the bond. Oxides foul the integrity of the bond. In addition, diffusion bonds are plagued with microvoids which are difficult to detect nondestructively, but, if present, significantly diminish the structural performance of the joint. Diffusion bonding also is a time consuming process. The part typically must be held at elevated temperature and elevated pressure (about 400 psi) for several hours. For example, in U.S. Pat. No. 3,920,175, the diffusion bonding operation takes five hours at 1650.degree. F. (900.degree. C.), making the forming/bonding operation six hours. In U.S. Pat. No. 3,927,817, diffusion bonding occurs prior to forming, still requires four to five hours, and forces a six hour bonding/forming cycle at 1650.degree. F. (900.degree. C.) for the entire period. Typically a hot press diffusion bonding process for common titanium alloys used in aerospace applications will require over eight hours at 2500 psi and 800.degree. C. (1472.degree. F.), about six hours at 400 psi and 900.degree. C. (1650.degree. F.), or about two hours at 250-300 psi and 950.degree. C. (1742.degree. F.). Producing this heat and pressure for this length of time is expensive.
The present invention is a timesaving process for an SPF/brazing cycle for multisheet parts that promises higher quality parts at lower production costs with significant energy savings in shorter production times. The troubles of diffusion bonding are eliminated by replacing diffusion bonding with brazing so that a much more efficient cycle is possible. Manufacturers have greater assurance in the integrity of the brazed bond, can achieve a satisfactory brazed bond quickly and reliably, and can process the multisheet pack with a single heating cycle without removing the pack from the press.